Cation exchange membranes

ABSTRACT

The invention provides cation exchange membranes made from sulphated ethylene-vinyl alcohol copolymers and a process for the preparation thereof.

United States Patent Marze 51 July 11, 1972 [54] CATION EXCHANGE MEMBRANES [72] Inventor: Xavier Maize, Lyon, France [73] Assignee: Rhone-Poulenc S.A., Paris, France [22] Filed: July 17, 1969 [21] Appl. No.: 842,718

[30] Foreign Application Priority Data July 18, 1968 France ..68159720 [56] References Cited UNITED STATES PATENTS 3,388,080 6/1968 deKiirosy ..204/296 X Field of Search ..204/180 P, 296

US. Cl ..204/296, 204/180 P 5 Chem. Abs. 64: 8417b 1966 Chem. Abs. 53: 95l9d 1961 Fogle ..204/296 X Japan Primary ExaminerJohn H. Mack Assistant Examiner-R. J. Fay

Att0rneyCushman, Darby & Cushman [57] ABSTRACT The invention provides cation exchange membranes made from sulphated ethylene-vinyl alcohol copolymers and a process for the preparation thereof.

5 Claims, No Drawings CATION EXCHANGE MEMBRANES transfer (quantity of solvent crossing the membrane for a given quantity of salt exchanged). When the membranes are used in other ways some or all of these properties are generally required.

The present invention provides cation exchange membranes which have valuable properties in electrodialysis as well as other properties such as insolubility in both water and the common solvents, resistance to alkaline reagents, good mechanical properties, flexibility even in the dry state, and no adhesive ability.

The cation exchange membranes of the present invention comprise a copolymer of formula:

wherein R is a hydrocarbon radical, n, m and p are positive integers and q is a positive integer or O and n/m+p is 4.5 to 60 and qlm+p+q is less than 0.1 R is preferably a lower alkyl radical containing 1 to 4 carbon atoms such as methyl and the ratio of n to m+p and of q to m+p+q are preferably 5.5 to 18 and below 0.04 respectively.

The membranes of this invention may be prepared by treating a film of a binary ethylene-vinyl alcohol copolymer and/or a ternary ethylene-vinyl alcohol-vinyl ester copolymer in which the ratio of ethylene/hydroxyethylene units is 4.5:] to 60:l, preferably 5.5:1 to 18:1, with one or more of sulphuric acid, oleum, chlorosulphonic acid, sulphur trioxide and the addition products of sulphur trioxide and a base.

The film is treated in a sulphuric bath, i.e., a liquid medium containing one or more of sulphuric acid, oleum, chlorosulphonic acid (ClSO l-l), sulphur trioxide, and the addition products of sulphur trioxide and bases, and optionally up to 95 percent, preferably less than 85 percent by weight of the total weight of the bath of a solvent.

Many diverse solvents may be used and they may even react with the sulphuric compound such as acetic acid which at least partially forms acetyl sulphate in the presence of chlorosulphonic acid. There are however some combinations of solvent and sulphuric compound which may not be used because the latter is then rendered useless for example water and chlorosulphonic acid.

The solvents which may be used, include liquid S monocarboxylic acids containing 1 to carbon atoms, chlorinated aliphatic hydrocarbons, such as chloroform, carbon tetrachloride and tetrachloroethylene, ethers and carbon disulphide.

When the sulphuric compound used is the addition product of S0 and a base, the solvent used can also be an excess of the base, for example a tertiary nitrogen base such as pyridine or a picoline, or a phosphine oxide, dioxane, or an unsubstituted or chlorinated aliphatic or aromatic hydrocarbon.

When sulphuric acid is used, the solvent used may also be water (in small amount) or a hydrocarbon.

The binary ethylene-vinyl alcohol copolymer and/or the ternary ethylene-vinyl alcohol-vinyl ester copolymer are hereinafter referred to as the ethylene-alcohol copolymer. Preferred ethylene-alcohol copolymers have a melt index (measured according to Standard ASTM D 1238 52 T) of less than 500 after complete acetylation.

The ethylene-alcohol copolymer films are obtained by known methods. Usually, an ethylene-alcohol copolymer is first prepared by the complete or partial saponification of an ethylene-vinyl ester copolymer and the ethylene-alcohol copolymer thus obtained is then formed into a film.

The type of the vinyl ester in the ethylene-vinyl ester copolymer used in the saponification is not critical, and may for example be vinyl forrnate, acetate, propionate, butyrate, stearate, benzoate, cyclohexanoate, isobutyrate, palmitate, myristate, toluate, naphthoate, campholate, acrylate or chloracetate. However, it is generally preferable to use the acetate.

It is of advantage to saponify the ethylene-vinyl ester copolymer until an ethylene-alcohol copolymer having a ratio of hydroxyethylene/hydroxyethylene acyloxyethylene, units greater than 0.90 and preferably greater than 0.96 is obtained.

Ethylene-alcohol copolymers are usually formed into a film by pressing when hot, or by casting the solution followed by evaporation. Cold hexamethylphosphotriamide, hot aromatic hydrocarbons such as benzene, toluene or xylene, or polar solvents such as dimethylfonnamide can for example be used as solvents in this casting. Films can be made of various thicknesses but they are usually 0.05 to 1 mm. thick.

Finally, to improve the mechanical properties of the membranes of this invention it is often advantageous to reinforce the ethylene-alcohol copolymer film by incorporating a reinforcing support, such as a screen, grid or woven fabric, into it during its preparation.

Using these techniques and in particular by casting the solution it is possible to prepare films of different geometric shapes, for example bag or tube form. However the membranes prepared are in general fiat.

Treating the ethylene-alcohol copolymer film in a sulphuric bath in the process of this invention consists in practice of immersing the film in the bath and allowing it to remain under these conditions until the required number of acid groups have become attached.

The required number of groups depends on the intended use of the membrane but there are usually 0.5 to 4 preferably 1 to 2.5, milliequivalents of -SO,,H groups per gram of dry membrane.

The film is generally treated in the sulphuric bath at a temperature of 20 C. to 120 C. and preferably of 40 C. to C.

The treatment time varies within wide limits depending on the desired degree of acidity of the membrane and on the temperature of treatment and above all on the composition of the sulphuric bath. Very short treatment times make it difficult to control the reaction and the reproducibility of the membranes and very long treatment times are of no value for economic reasons. Normally the treatment lasts for 10 minutes to 15 hours, preferably for l to 8 hours.

The treating of the ethylene-alcohol copolymer film in the sulphuric bath can be carried out continuously in the case of indefinitely long membranes, or discontinuously in other cases. This may be followed by various washing operations to eliminate particularly the non-macromolecular material in the membrane.

The membranes of this invention can either be used as such or else they can be further treated with a bleaching agent, such as an aqueous alkaline solution containing active chlorine, thus improving their electro-chemical properties and in particular lowering their electrical resistance. The composition of the bleaching agent is not critical as the electrical resistance of the membrane is lowered by contact with any aqueous solution containing OH and C10 ions. In practice the membrane is treated with the bleaching agent until it has a constant electrical resistance, which may readily be determined by measurements on samples of the membranes.

The treatment of the membrane with a bleaching agent can be carried out with heating, but it is in general preferable to work at ambient temperature to avoid deterioration in the mechanical properties of the membrane. The treatment of the membrane with a bleaching agent can be followed by various washing operations.

The following Examples illustrate the invention.

The properties of the prepared membranes were determined by carrying out the following measurements.

a. Degree of Acidity (expressed in milliequivalents per gram (except in Examples 4 and 20 where a hot solution of the ethylene-alcohol copolymer in xylene was cast and the solvent then evaporated). The thickness of the prepared film is 0.2 mm (except in Example 1 where it is 0.1 mm).

of dry material). The film is immersed in a hot sulphuric bath consisting of a This is determined by neutralization with an alkali solution 23 percent by weight solution of ehlorosulphonic acid in of known strength. acetic acid.

b. Electrical Substitution Resistance (i.e. the change for a Af er r i n, h m n Obtained i washed w h given membrane surface in the electrical resistance ofa liquid cessi e aqueous solut ons of sulphuric aci of conce ration cylinder inadirection perpendicular to the axis of the cylinder decreasing from 70 ercent to 0 per n (P Water) y when the membrane is substituted for a slice of liquid of the W gh same thickness and ofthe same surface as the membrane). In Ex mp 8 n membranes p p r ing to In the present case the substitution resistance is measured in Examples 7 and 14 respectively were treated in addition with an aqueous solution of 0.6 M KCl and is expressed in 0 cm sodium hypochlorite. This was done by leaving the memc. Selective Permeability (i.e. the ability of the membrane to 15 branes for 24 hours at C. in a solution of sodium prevent the passage of cations while allowing that of anions. hypochlorite of about 48 chlorometrlc degrees (commercial The selective permeability is calculated from a mea ureconcentrated bleach solution containing 2.14 moles of sodium ment of the electromotive force, E, between an 0.4 and an 0.8 hyp ch P M aqueous solution of KCl separated by the embra i The conditions of the preparation of the membranes used in question, which has previously been saturated with an 0,6 M 20 each Example and the results obtained in each case are shown aqueous solution of KCl. in the following Table:

Melt index Degree of the comof acid- Substi- Ethylene/ pletely Immerity of tution vinyl acetylated sion Sulphuric the memresistance Selective alcohol othylene/ time hath brnno of tho permeamolecular alcohol of film tomporanlogJg. membrane bility ratio copolymer (hours) turo 0.) Special features of preparation of dry (ncnifi) (percent) 6. 2 5 0.3 48 6. 2 25 (1 1 68 6. 2 25 7 7 80 7. 9 150 4 6 82 7. 9 150 4 7 79 7.9 150 7 4. 3 79 7. 9 25 4 13 89 7. 9 25 4 6 83 7. 9 25 3 3 84 7. 9 15 2% 1 79 7.9 15 3 5 s7 7. 9 15 4 6 s7 7. 9 15 7 6. 5 84 7. 9 5 4 37 95 7. 9 6 4 13 90 7.9 6 4% 1. 4 4 86 7. 9 G 5 1. 5 2. 5 s3 9. 2 400 2%,, 1. 2 5 79 i). 2 2 4 0 1. 9 4 90 14 150 6 85 Film obtained by casting 1. 9 3. 5 (l7 The formula used to calculate the selective permeability, P, The water transfer of the membrane of Example 5 is 6 and as a percentage is: 5 that of the membrane of Example 19 is 12.2.

P The membranes of Examples 13 and 19 were in addition 4 tested for their resistance to alkaline reagents by immersing 'them for 24 hours at 20 C. in a normal aqueous solution of in which t is the transport number of K in an aqueous solupotassium hydroxide. Their substitution resistance and their tion of 0.6 M KCl and t the transport number of K in the selective permeability were not affected by this treatment. membrane. All the membranes obtained in these 20 Examples have t is calculated using the formula: good mechanical properties, are flexible even in the dry state, t'' (E E0)/2 E0 in which E0 (RT/F) ln (al/a2) show no adhesive ability and are insoluble in water and comwhere E is the electromotive force between the 0.4 M and the mon solvents. 0.8 M aqueous solution of KCl, R is the gas constant, T is the Iclaim: absolute temperature, F is one Faraday (96,489 coulombs per 1. A cation exchange membrane which comprises a gram equivalent, a1 is the activity of the more concentrated copolymer of the formula: electrolyte (calculated from the concentration of the eleccH. CH (CH CH)m(-C 2 )n( zCH) trolyte and the actlvlty coefficient) and a2 is the actlvlty of the S O H 0 less concentrated electrolyte. Y 4 h C0 R d. Water Transfer (only measured in Examples 5 and 19). wherein R is a hydrogen atom or a methyl, ethyl, propyl, hex- The amount of water which has migrated across a memadecyl, phenyl, cyclohexyl, isopropyl, pentaclecyl, tridecyl, brane dividing a cell into two compartments, one compartphenylmethyl, naphthyl, l,2,2,3-tetramethylpentyl, vinyl or ment containing pure water, the other containing an aqueous chloromethyl radical, n, m and p are positive integers and q is solution of 1.2 M KCl, is measured. This water transfer is exa positive integer or 0 and n/m-i-p is 4.5 to 60 and q/m+p+q is pressed in mm per hour per cm of membrane for a difference less than 0.1. of l mol/liter between the concentrations of the two solutions. 2 A membrane accofding to claim 1 wherein n/m+p is 5 5 EXAMPLES 1 to 20 3. A membrane according to claim 1 wherein qlm+p+q 15 A series of experiments to prepare membranes correspondl h ()4 g to formula is arried out by the following general 4. A membrane according to claim 1 wherein the copolymer method: contains 0.5 to 4 milliequivalents of SO H groups per gram An ethylene-vinyl acetate copolymer is saponified to a fd membrane,

degree of saponification greater than 95 percent. The 5. A membrane according to claim 1 which also comprises a ethylene-alcohol copolymer obtained is hot pressed into a film remforcmgsupport. 

2. A membrane according to claim 1 wherein n/m+p is 5.5 to
 18. 3. A membrane according to claim 1 wherein q/m+p+q is less than 0.04.
 4. A membrane according to claim 1 wherein the copolymer contains 0.5 to 4 milliequivalents of -SO4H groups per gram of dry membrane.
 5. A membrane according to claim 1 which also comprises a reinforcing support. 